The obtaining of optical elements such as glass lenses by precision mold pressing is known. The optically functional surfaces that are formed by press molding afford desired optical performance without requiring mechanical processing such as polishing.
In optical glass elements such as lenses obtained by precision mold pressing, the optically functional surfaces (which are often aspherical) obtained by press molding are employed without further mechanical processing. However, the outer perimeter portion of the molded product is often removed to adapt it to the optical apparatus on which it is being mounted. For example, polishing the outer peripheral surface to define the outer perimeter and aligning the center of the outer perimeter with the optical axis are common practices. Further, as needed, the vicinity of the outer rim of the first and/or second surface of a molded product is often polished to form a flat surface perpendicular to the optical axis. So doing permits accurate positioning of the lens relative to the optical apparatus in which it is mounted.
Optical glasses of low refraction and low dispersion are extremely useful as the materials of lenses constituting optical systems such as image pickup apparatuses. In particular, optical glasses having optical constants such as a refractive index value nd (1) of 1.40 to 1.60 and an Abbé number (v(nu)d) of 67 or higher are useful for eliminating chromatic aberration in zoom lenses and the like. The present inventors developed a glass having such optical constants in the form of a fluorophosphate glass having a new composition. This fluorophosphate glass is a high value-added glass capable of achieving the above-stated optical characteristics. The present inventors also studied the use of this fluorophosphate glass in optical glass elements obtained by precision mold pressing.
Based on investigation conducted by the present inventors, optical glasses having the above-stated optical constants permit optical elements with relatively good mold release properties and high precision in press molding. However, the present inventors determined for the first time that these glasses tended to have high degrees of abrasion, and presented a problem in the form of a tendency to scratch during handling after shaping by mechanical processing such as that set forth above.
Further, optical glasses with high refractive indexes (such as an nd of 1.7 or higher) and high dispersion (such as a v(nu)d of 35 or lower) are highly valued for eliminating chromatic aberration in small image pickup apparatuses and the like, so the need is great. The present inventors developed a phosphate glass with a high refractive index that was capable of responding to these demands. However, this glass also tended to have a high degree of abrasion. As a result, just as with fluorophosphate glass, it was newly revealed that it had a problem in the form of a tendency to scratch during handling after shaping such as mechanical processing.
However, since fluorophosphate glasses and phosphate glasses such as those set forth above are novel, there was no previously known countermeasure to scratching caused by handling after shaping such as mechanical processing of fluorophosphate glasses and phosphate glasses such as those set forth above.
Investigation by the present inventors revealed that those optical glasses having a degree of abrasion of 200 or higher were the ones that tended to scratch during handling after shaping such as mechanical processing. During mechanical processing, particularly during a step known as centering and edging, in which a lens is gripped between opposing bell members and the outer perimeter of the pressed lens is polished with a grindstone, scratches tend to form on the optically functional surfaces of the lens that is being gripped and pressed. These scratches have been found to affect optical performance.
The forming of a coating on the surface of a glass material obtained by press molding is known. For example, Patent Document 1 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 08-277125, the entire contents of which are hereby incorporated herein by reference) describes a method of forming a coating of a group IIIa metal oxide on the surfaces of a glass lens molding material on which functional surfaces are to be formed. It is disclosed that so doing prevents fusion between the molding material and the surface of the mold.
Patent Document 2 (Japanese Unexamined Patent Publication (KOKAI) Showa No. 62-226825, the entire contents of which are hereby incorporated herein by reference) describes a method whereby a fusion-preventing layer is inserted between a glass member being molded that has a shape constituting the foundation of the finished shape of a lens and a pressing mold comprised of a glass having a higher glass transition temperature than the pressing temperature, and the glass member is press molded by the pressing mold while in a softened state.
Above-cited Patent Documents 1 and 2 describe the formation of a film on the surface of the glass material as a means of preventing fusion of the glass material and the pressing mold. However, they both have as their objective to prevent fusion between the glass material and the pressing mold, and do not seek to prevent scratching of press-molded glass products caused by handling after shaping by mechanical processing or the like.
Accordingly, the present invention has for its object to provide a means of solving the above-stated problems, whereby an optical element does not bear scratches on optically functional surfaces through contact during handling after the press molding of press molded glass products despite being comprised of a glass material with a high degree of abrasion, and a means whereby even when scratched, the function of the optical element finally obtained is unaffected.